Activation of Endoplasmic Reticulum-Localized Metabotropic Glutamate Receptor 5 (mGlu5) Triggers Calcium Release Distinct from Cell Surface Counterparts in Striatal Neurons

While G protein-coupled receptors (GPCRs) have traditionally been viewed as signaling exclusively from the cell surface, recent evidence suggests they can also function from internal membranes. This study investigates the functional role of metabotropic glutamate receptor 5 (mGlu5) specifically localized on the endoplasmic reticulum (ER) in striatal neurons, testing whether this intracellular pool operates independently of surface receptors to shape calcium signaling.

To isolate ER-specific signaling, the researchers used a specialized ER-targeted calcium sensor, ER-GCaMP6-150. By employing specific pharmacological tools—using permeable versus non-permeable agonists (e.g., Quisqualate vs. DHPG) and various receptor antagonists—the team distinguished between the calcium release triggered by surface-bound versus ER-localized receptors. They further probed the underlying signaling pathway by using inhibitors for Gq proteins, phospholipase C (PLC), and calcium release channels (IP3 and ryanodine receptors) to characterize the molecular mechanism of this internal calcium mobilization.

The authors demonstrate that ER-localized mGlu5 receptors are not merely inactive stores but active signaling units. When activated, these receptors generate rapid, pronounced drops in ER calcium levels, significantly more robust than the transient responses initiated by cell surface mGlu5 receptors. This process follows the canonical Gq/11-PLC-IP3 signaling pathway, which is distinct from the mechanisms utilized by surface receptors. Furthermore, the findings show that NMDA receptors do not contribute to these specific ER calcium dynamics in this model, confirming that the observed effects are primarily driven by the ER-localized mGlu5 pool.

Understanding that mGlu5 receptors function as autonomous signaling platforms at the ER reveals a deeper layer of complexity in neuronal calcium regulation. Because calcium signaling is fundamental to synaptic plasticity, neurodevelopment, and cellular homeostasis, this compartmentalized signaling may allow for more precise control over local neural activity than previously understood. This study provides crucial evidence that mGlu5 is a dynamic player at multiple membrane locations, potentially broadening the scope for therapeutic targeting in neurological disorders.